Not long ago metal halide/HQI lighting was
the lighting of choice for experienced reef keepers because of their
high PAR values which generally exceeded 300+. With the recent
advent of LED lighting technologies, many hobbyists were leery about LED
lighting because of their seemingly low PAR values and apparent lack of
brightness. To understand the reasoning for this, you must first
understand the spectral wavelength and how it affects coral growth, our
eyes, and the technology behind Orphek LED lighting for reef
systems.

The acronym PAR relates to Photosynthetic
Available Radiation. This is all light that humans can see and
covers a range between 400-700 nanometers. All light in this
spectrum range emits radiation and is how wavelengths are measured
(nanometers). The radiation in this range is not all useful light
for coral growth, and in fact, just a small percentage of it.
There are two factors in this range that affect the human eye;
brightness and chromacity. As an example, the color white is a
bright color, while the color grey is considered to be a less bright
version of that same white. In other words, the chromaticity of white
and grey is the same while their brightness appears different to our
eyes. The reason for this is that the human eye is more sensitive
to brightness in certain colors. The yellow and green spectrum is
especially dominant which means that green will appear much brighter
than the same light intensity of light in the blue spectrum. This
is where the common misconception lies among aquarists regarding LED
lighting; “the light isn’t as bright as metal halides”.

The acronym PUR relates to
Photosynthetic Usable Radiation. This is the spectral range of
light that is most beneficial to coral growth and has two ranges;
400-550 and 620-700. This also happens to be in the spectrum ranges
where light intensity is the least sensitive to our eyes.

The reason high PAR values or
intensity is achieved with metal halide lighting is that Quantum meters
measure the light being emitted throughout the visible spectrum of light
(400-700nm) and much of this light is not useable for coral growth.
The actual PAR values of the useable light (PUR) would be more like
100-150.

Orphek’s white LED technology allows more white light to be used and
less blue. The benefit of this technology is more lumens per watt
which significantly raises PAR/PUR values and allows excellent coral
growth with less heat and wasted energy. Some companies use
standard off the shelf Cree LEDs which have a higher lumens per watt and
a shift toward the green spectrum which makes them appear bright.
Unfortunately, the only use green has is to promote nuisance algae
growth in your system. The graphs below compare Orphek’s
16,000K white LED with a Cree 7,000K LED. You can see in the
chromacity chart (bottom right) where the Cree LED has a large color
shift toward green and a color rendering index (CRI) of 75.

Orphek’s white LED
technology allows our LEDs to produce light that mimics 18,000K while
producing very high PUR per watt levels. This is why it is very
important to view a spectrograph of a LED fixture you intend to
purchase. As the above Cree graph depicts, there is much wasted
energy between 500 and 580nm (about 50%) as this spectrum of light is
not of much use for coral growth and lowers the needed PUR value.
This is why PAR values can be misleading without understanding
the spectrum and spectrographs. The Cree white LED spectrograph
would likely result in a higher lumens per watt ratio than the
Orphek white LED, but not when you mix one Cree LED and one Cree
blue LED to get the 18,000K look. This is due to the fact that
blue LEDs have a very low lumens per watt ratio. In the Cree
spectrograph there is wasted energy in wavelengths (500-575nm) that are
of no use for coral growth. So if we compare the Orphek
spectrograph to the Cree, the Cree would result in a higher lumens per
watt value. It is understandable why some companies use the Cree
LEDs and it’s because of their high lumens per watt ratio. It must
be understood that an excessive shift toward the green spectrum results
in a lower PUR level. The Orphek spectrograph clearly illustrates that
our LEDs are more beneficial for coral growth because the wavelengths
are tuned to the light spectrum (PUR) that benefit corals and not other
wavelengths that are of no use for growing corals. High intensity
in the green and yellow spectrum negatively affects coral and anemone
growth. The Orphek chromacity graph shows a shift to blue and red
(chlorophyll A and B) which is beneficial for coral growth.

It cannot be stressed enough that high PAR
values in LED lighting fixtures do not necessarily mean the light is
going to be better for growing corals.